The present invention relates to a semiconductor device that includes a MOS transistor having a gate electrode or interconnection formed as a layer of a refractory metal silicide deposited over a polysilicon layer, and, in particular, to such a semiconductor device that can be used in an extremely small transistor device.
As the degree of integration of semiconductor devices increases, it is becoming more common for design rules to lie in the so-called submicron region, for example, at 0.7 .mu.m. The structure of an electrode or interconnection of this type of semiconductor device is as shown in FIG. 4C. A gate electrode or interconnection 405 is formed of a polysilicon layer 402 doped with an impurity by a method such as phosphorous diffusion in a region surrounded by an element isolation oxide film 406 on a silicon substrate 401, a refractory metal (high melting point metal) silicide layer 403 of tungsten or molybdenum is deposited over the polysilicon layer 402, and subsequently a post-oxidation film 404 is formed thereon.
This structure is formed as follows. First, the field oxide film 406 used to isolate neighboring elements is formed on the silicon substrate 401 by a selective oxidation method such as LOCOS, using a silicon nitride film as an oxide-resistant film, and a gate oxide film 407 is formed by thermal oxidation on the surface of the silicon substrate 401 in a region surrounded by the field oxide film 406. A polysilicon layer 408 is then deposited over the entire surface, and an impurity such as phosphorous is implanted and diffused thereinto to obtain a polysilicon layer doped to a high concentration with an impurity (FIG. 4A).
Next, a refractory metal silicide layer 409 is deposited over this impurity-doped polysilicon layer 408, a photoresist layer 410 formed on the silicide layer 409 is exposed and developed, and this is used as an etching mask to selectively remove the polysilicon layer 408 and the silicide layer 409 (FIG. 4B) to obtain the patterned polysilicon layer 402 and the refractory metal silicide layer 403. Oxidation is then performed to complete the gate electrode or interconnection 405 by forming the post-oxidation film 404 surrounding the polysilicon layer 402 and the refractory metal silicide layer 403 (FIG. 4C).
However, the impurity concentration of the polysilicon layer of this semiconductor device having a gate electrode or interconnection is much higher than that of the silicide layer, so the oxidation rate of the polysilicon layer directly under the silicide layer is higher during the post-oxidation, and oxidized portions called bird's beaks are generated thereby, as denoted by 410 in FIG. 4C. These bird's beaks exert stress on the refractory metal silicide layer, which is a cause of peeling of the refractory metal silicide layer in the processing steps subsequent to post-oxidation. This peeling of the refractory metal silicide layer greatly increases the resistance of the gate electrode or interconnection, leading to defects of the device.
Experiments have shown that no peeling of the silicide layer due the above bird's beaks occurs during subsequent processing in a device in which an impurity is thinly implanted into the polysilicon layer under the conditions of phosphorous diffusion for 30 minutes in an atmosphere at 850.degree. C., but such peeling of the refractory metal silicide layer due to bird's beaks does occur in a device in which ions are implanted thickly into the polysilicon layer by implantation of As.sub.+ ions at a dosage of 1.times.10.sup.15 cm.sup..times.2 and an acceleration energy of 60 KeV in addition to the phosphorous diffusion for 30 minutes in an atmosphere at 850.degree. C., and also in a device in which an impurity is thickly implanted into the polysilicon layer under the condition of phosphorous diffusion for 60 minutes in an atmosphere containing POCl.sub.3 gas at 850.degree. C.
In the above element in which peeling of the refractory metal silicide layer due the above bird's beaks does not occur, the impurity concentration increases the resistivity of the gate or causes depletion in the region of the gate oxidation layer in the polysilicon, so it is preferable that the impurity concentration of the polysilicon layer should be maintained at a high level.
As described above, the semiconductor device of this conventional structure has the problem that bird's beaks are generated during post-oxidation in the boundary portion between the polysilicon layer and the silicide layer.